Formation of a metalorganic compound for growing epitaxial semiconductor layers

ABSTRACT

Semiconductor devices are prepared by growth of epitaxial layers on a substrate from metalorganic compounds of the formula MR 3 , R being an alkyl group, or its amine adduct. The metalorganic compound was prepared by reacting a Grignard reagent with a metal halide in an amine solvent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International PatentApplication No. PCT/GB95/02089, filed Sep. 4, 1995; which claimspriority from Great Britain Application GB94107707.8, filed Sep. 2, 1994and Great Britain Application GB9508702.9, filed Apr. 28, 1995.

BACKGROUND OF THE INVENTION

This invention concerns semiconductor devices and their production.

While there is a considerable interest in InP/InGaAs devices grown bychemical beam epitaxy (CBE), particularly in the area of selectivegrowth, the GaAs/AlGaAs material system is less well established. Withthe exception of very encouraging data for heterojunction bipolartransistors (HBT's) and high eletron mobility transistors (HEMTS's) thequality of AlGaAs alloys grown by CBE have generally been inferior tothose grown by metalorganic vapor phase epitaxy (MOVPE) or molecularbeam epitaxy (MBE). The degradation of material quality results fromunintentional moieties of group III metal alkyls and also unintentionaloxygen incorporation. Much effort has, therefore, gone into thedevelopment of new precursors which reduce unintentional impurityincorporation in epitaxial AlGaAs layers.

A direct correlation has been established between oxygen concentrationsunintentionally incorporated into AlGaAs grown by CBE and tracequantities of diethylether detected by in-situ modulated beam massspectrometry (MBMS) in the group III metalorganic precursors. The traceether is residual from the synthesis of the metal trialkyl MR₃, whichinvolves the alkylation of the metal trihalide by a Grignard reagentRMgX carried out in an ether solvent. Subsequent purification processesare then performed to remove the oxygen containing ether solvent andother impurities from the metalorganic precursor. However, theseprocesses are never entirely successful.

For example, U.S. Pat. No. 4,464,233 describes the formation ofdimethylmagnesium by reacting a Grignard reagent with a metal halideusing electrolysis, having tetra(n-butyl) ammonium percholate as anionizable support electrolyte and a solvent such as an aliphatic ether,cyclic aliphatic mono- or poly- ether or a non-cyclic ether. Similarly,U.S. Pat. No. 4,604,473 discloses a method of producing atrialkylgallium compound by reacting a gallium trihalide with a Grignardreagent in the presence of an ether.

Trimethylindium compounds with nitrogen-containing Lewis-bases have alsobeen prepared using Lewis-base solvents, such as diethyl ether (seeJournal of the Chemical Society, Dalton Transactions vol. 1. 1998, USA;Foster et al: "synthesis and thermal properties of trimethylindium withnitrogen-containing Lewis bases") and the interaction of, for example,ethers and amines with trimethylalane adducts is discussed in InorganicChemistry vol. 7, no. 6., Jun. 3, 1986, USA pages 1047-1051; C. H.Henrickson et al.

Alternative methods for producing metalorganic precursors for use in thedeposition of epitaxial layers have been described in, for example, U.S.Pat. No. 4,812,586 and EP0460598.

BRIEF SUMMARY OF THE INVENTION

A first object of this invention is to provide a method of growingsemiconductor layers in which the above-mentioned oxygen impurityproblem is eliminated or at least reduced in effect.

A second object of this invention is to provide semiconductor devices inwhich the above-mentioned oxygen impurity problem is eliminated or atleast reduced in effect.

According to a first aspect of the invention there is provided a methodof growing semiconductor layers on a substrate comprising the steps ofdelivering to the substrate, a metalorganic compound of the formula MR₃,R being an alkyl group, prepared by reacting a Grignard reagent with ametal halide, and causing deposition on the substrate of metal from themetalorganic compound, characterised in that reaction of the Grignardreagent with the metal halide is carried out in an amine solvent.

According to a second aspect of the invention there is provided asemiconductor device comprising a layer grown on a substrate from ametalorganic compound of the formula MR₃, wherein the metalorganiccompound is prepared by reacting a Grignard reagent with the metalhalide characterized in that said reaction is carried out in an aminesolvent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of vapor pressure versus source temperature forpreparation of a tri-isopropyl gallium adduct according to the method ofthe invention (solid line) and by a conventional method (dashed line);and

FIG. 2 is a plot of growth rate versus bath temperature for preparationof a tri-isopropyl gallium adduct according to the method of theinvention (solid line) and by a convention method (dashed line).

DETAILED DESCRIPTION OF THE INVENTION

The Grignard reagent for use in preparing the metalorganic compound ispreferably prepared in an amine solvent, especially the amine to be usedin preparing the metalorganic compound.

The amine is preferably a tertiary amine such as, for example, atertiary alkyl amine or a tertiary heterocyclic amine. Amines for use inthe preparation of the metalorganic compound and/or the Grignard reagentare preferably liquid at room temperature typically 18 to 20° C.Preferred tertiary alkyl amines have the formula ##STR1## wherein R¹, R²and R³ are alkyl groups having from 1 to 4 carbon atoms and wherein R¹,R² and R³ may be the same or two of R¹, R² and R³ may be the same.Preferred amines for use in the invention are triethylamine anddimethylethylamine.

Suitable heterocylic amines for use in the invention may includepyridine, 2H-pyrrole, pyrimidine, pyrazine, pyridazine, 1,3,5-triazineand hexahydrotriazine.

The process for preparing the metalorganic compound can result in anamine adduct which may be used in the method of the invention instead ofthe metalorganic compound alone.

Suitable metalorganic amine adducts have the formula

    MR.sub.3.A

wherein M is a metal, R is an alkyl group and A is a tertiary amine suchas a tertiary alkyl amine of the formula ##STR2## wherein R¹, R² and R³are as defined above or a tertiary heterocyclic amine.

The Grignard reagent may be prepared in any suitable way, typically byreaction of magnesium with an alkyl halide, wherein the alkyl group isthat required for the metalorganic compound.

The preparation of metalorganic compounds and metalorganic amine adductspreferably follows the following sequence of steps:

1. Synthesis of RMgX in NR₃ solvent;

2. Suspension of MCl₃ in pentane;

3. Addition of RMgX to MCl₃ in NR₃ /pentane;

4. Removal of volatiles and isolation of MR₃ (NR₃) by distillation;

5. Removal of volatile impurities from MR₃ (NR₃);

6. Isolation of the adduct or thermal dissociation of MR₃ (NR₃) andremoval by fractional distillation of the NR₃ ligand.

Metalorganic compounds that may be useful in the invention include alkylcompounds of Group II, Group III and Group V metals. Examples of suchcompounds include dialkyl zinc, dialkyl cadmium, trialkyl aluminium,trialkyl gallium, trialkyl indium, trialkyl phosphorus, trialkyl arsenicand trialkyl antimony.

The formation of an amine permits the removal of volatile metallic andnon-metallic microimpurities from the metalorganic compound. Impuritiesmay be readily removed from the adduct by distillation. The adduct maybe split by removal of the amine, such as by heating, to provide themetalorganic compound alone for some purposes, such as a precursor forMOVPE or CBE. Alternatively the adduct itself may be used as a precursorfor the deposition of, for example Group III-V or II-VI layers, such asgallium arsenide, aluminium gallium arsenide and zinc selenide, byMOVPE, CBE and other vapour phase epitaxy techniques.

In general the method of the invention may be used for the growth ofGroup III-V semiconductor device structures. The method of the inventiongenerally encompasses all vapour phase growth techniques, such as, forexample; MOVPE, CBE and MOMBE.

The method of the invention may be used in the production ofsemiconductor devices for electronic applications e.g. transistors (FET,HEMT and HBT devices), diodes, superlattice current emitters and mixerdiodes. Photonic application devices may also be produced e.g. lasers(vertical cavity (VCSEL) and planar), light emitting diodes, reflectorsand modulators (including those used as part of the VCSEL,photodetectors, photodiodes and optical waveguides.

Integration of any of the above using selective growth techniques may beused to achieve lateral integration e.g. for lasers and waveguides.

The method of the invention may further be used to produce deposits inGroup II-VI devices for e.g. long wavelength infra red detectors andsources. The method of the invention may be further used to producemetal coatings e.g. for Al, Ga and In deposition.

AlGaAs layers may be produced by vapour phase deposition, especially bychemical beam epitaxy, of a trialkyl gallium amine adduct, typically oftrisopropyl gallium with an AlH amine adduct and arsine.

The method of the invention may be used in, for example, low temperatureorganometallic vapour phase epitaxy of InSb preferably usingtri-isopropyl antimony as the precursor. Typically tri-isopropylantimony and trimethylindium are used to grow InSb epilayers on GaAs.Similarly GaSb may be grown by MOVPE from tri-isopropyl antimony andtrimethylgallium.

In one preferred method of the invention the metalorganic compound isdelivered under high vacuum to a substrate without carrier gas andpyrolysed on the substrate. The metalorganic compound or its amineadduct may be used. The substrate is typically a Group III to V singlecrystal such as of gallium arsenide. This method may be especiallysuitable for deposition of Group III metal especially gallium andespecially from triisopropyl gallium.

Antimony may also be deposited on a substrate in a similar way but mayalso be deposited as may other metals from metalorganic compounds ortheir amine adducts using MOVPE techniques in which a carrier gas isused to transport the metalorganic vapour at pressures typically in therange of 2 to 760 Torr to the substrate, such as a single crystal GroupIII to V substrate, where it is pyrolysed to form the desired layer.

Typical semiconductor devices which may be made using the inventioninclude CD lasers and high speed transistors.

The invention will now be further described by means of the followingexamples. Each reaction described below was carried out in an atmosphereof dry/oxygen-free dinitrogen using reagents which had been dried anddeoxygenated by standard purification methods.

EXAMPLE 1

This example demonstrates the production of triisopropylgallium usingtriethylamine as solvent.

A solution of iso-propyl magnesium bromide, i-PrMgBr, in triethylaminewas prepared by the dropwise addition of iso-propyl bromide, i-PrBr (280g, 2.3 mol) to a stirred suspension of magnesium metal turnings (60 g,2.5 mol) in triethylamine, NEt₃ (1000 cm³). This resulted in a vigorousexothermic reaction. It was found that this reaction could be moreeasily initiated by the addition of a crystal of iodine. After completeaddition of the i-PrBr, the reaction mixture was stirred at ambienttemperature for 4 hours.

A solution of gallium trichloride, GaCl₃ (125 g, 0.7 mol) in pentane(500 cm³) was then added slowly with stirring to the solution ofi-PrMgBr in NEt₃. This led to an exothermic reaction. After completeaddition of the GaCl₃ -pentane solution, the reaction mixture wasstirred for 4 hours at room temperature to ensure complete reaction.

After removal of volatiles by distillation in vacuo, the crude productwas isolated by vacuum distillation (100° C.) into a receiver cooled inliquid nitrogen (ca -196° C.). Volatile impurities were removed from thecrude product by distillation in vacuo (25-50° C.) and the pure liquidproduct was obtained by vacuum distillation (80° C.) into a cooledreceiver (ca -106° C.).

The metalorganic product was identified using proton NMR spectroscopy asa triethylamine adduct of triisopropylgallium, i-Pr₃ Ga(NEt₃)₀.6.

The proton NMR data are summarised below:

    ______________________________________                                        (ppm)                (Assignment)                                             ______________________________________                                        0.8 (triplet, 5.4H)  NCH.sub.2 CH.sub.3                                         1.0 (multiplet, 3H) GaCH(CH.sub.3).sub.2                                      1.4 (doublet, 18H) GaCH(CH.sub.3).sub.2                                       2.4 (quartet, 3.6H) NCH.sub.2 CH.sub.3                                      ______________________________________                                    

The i-Pr₃ Ga-NEt₃ adduct was further analysed for trace metal impuritiesusing inductively coupled plasma emission spectroscopy (ICP-ES). Theonly impurities detected were silicon (0.03 ppm w.r.t. Ga) and zinc (0.2ppm w.r.t. Ga).

Yield i-Pr₃ Ga(NEt₃)₀.6 =49.4 g.

The vapour pressure of the iPr₃ Ga adduct was found to be 0.9 mBar ar13° C.

The tri-isopropyl gallium prepared in the above way was used to grow alayer of AlGaAs on a gallium arsenide substrate by chemical beam epitaxyunder the following conditions:

Substrate temperature--540° C.

AlGaAs growth rate 1/hr

Group V precursor--thermally cracked arsine

Group III precursors--tri-isopropyl gallium triethylamine adduct plusAlH₃ -NMe₂ Et

An AlGaAs layer (aluminium composition of 18%) grown in this mannerdemonstrated oxygen levels of less than 4×10¹⁶ cm⁻³ (as measured bysecondary ion mass spectrometry, SIMS). This layer is superior to anAlGaAs layer (aluminium composition of 25%) grown usingtriisopropylgallium synthesised in a conventional manner (i.e. using anether solvent), and AlH₃ (NMe₂ Et), in which much higher oxygen levelsof 9×10¹⁶ cm⁻³ were detected by SIMS. The AlGaAs layer grown using thetriisopropyl gallium-triethylamine adduct was comparable in oxygencontent (<4×16 cm⁻³) with the best layers thus far obtained usingtriethylgallium and AlH₃ (NMe₂ Et) under identical CBE growthconditions.

FIGS. 1 and 2 respectively of the accompanying drawings show comparisonof vapour pressures and growth rates of the tri-isopropyl gallium adductprepared according to this Example and tri-isopropyl gallium prepared inthe conventional way. As can be seen the adduct has both higher vapourpressures and growth rates which are advantageous for chemical vapourdeposition processes.

EXAMPLE 2

This demonstrates the production of tri-isopropylgallium usingdimethylethylamine as solvent.

A solution of iso-propylmagnesium bromide, i-PrMgBr, indimethylethylamine was prepared by the dropwise addition ofiso-propylbromide, i-PrBr (166 g, 1.4 mol) to a stirred suspension of Mgmetal turnings (48 g, 2.0 mol) in dimethylethylamine, NMe₂ Et (500 cm³).This resulted in a vigorous exothermic reaction which could be moreeasily initiated by the addition of a small quantity of iodine. Aftercomplete addition of the i-PrBr the reaction mixture was stirred at roomtemperature for 4 hours.

A solution of GaCl₃ (69 g, 0.4 mol) in pentane (260 cm³) was then addedslowly, with stirring, to the solution of i-PrMgBr in NMe₂ Et. This ledto a vigorous exothermic reaction. After complete addition of the GaCl₃-pentane solution, the reaction mixture was stirred for 4 hours at roomtemperature to ensure complete reaction.

After removal of volatiles by atmospheric pressure distillation (60°C.), the crude product was isolated by vacuum distillation (100° C.)into a cooled receiver (ca -196° C.). Volatile impurities were removedfrom the crude products in vacuo, and the pure liquid product wasobtained by reduced pressure distillation (70° C.) into a receiver.

The metalorganic product was identified using proton NMR spectroscopy asthe dimethylethylamine adduct of triisopropylgallium, i-Pr₃ Ga(NMe₂ Et).The proton NMR data are summarised below:

    ______________________________________                                        (ppm)                (Assignment)                                             ______________________________________                                        0.6 (triplet, 3H)    NCH.sub.2 CH.sub.3                                         0.9 (multiplet, 3H) GaCH(CH.sub.3).sub.2                                      1.4 (doublet, 18H) GaCH(CH.sub.3).sub.2                                       1.9 (singlet, 6H) NCH.sub.3                                                   2.4 (quartet, 2H) NCH.sub.2 CH.sub.3                                        ______________________________________                                    

The i-Pr₃ Ga-NMe₂ Et adduct was further analysed for trace metalimpurities using ICP-ES. The only impurities detected were silicon (0.2ppm w.r.t Ga) and Zinc (4.6 ppm w.r.t Ga).

Yield i-Pr₃ Ga(NMe₂ Et)=58.5 g

EXAMPLE 3

This example demonstrates the production of tri-isopropylindium usingtriethylamine as solvent.

A solution of i-PrMgBr in NEt₃ was prepared by the dropwise addition ofi-PrBr (72 g,0.6 mol) in NEt₃ (200 cm³). This led to a vigorousexothermic reaction. After complete addition of the i-PrBr the reactionmixture was stirred at room temperature for 4 hours.

The solution of i-PrMgBr in NEt₃ was added dropwise, with stirring, to asuspension of indium trichloride, InCl₃ (35 g, 0.2 mol) in NEt₃ (200cm³). This led to an exothermic reaction. After complete addition of thei-PrMgBr/NEt₃ solution, the reaction mixture was boiled under reflux for2 hours.

After removal of volatiles by distillation in vacuo, the crude productwas obtained by vacuum distillation (100° C.) into a cooled receiver (ca-196° C.). Volatile impurities were removed from the crude product bydistillation in vacuo and the pure liquid product was obtained by vacuumdistillation (70° C.) into a cooled receiver (ca -196° C.).

The metalorganic product was identified using proton NMR spectroscopy asa triethylamine adduct of triisopropylindium, i-Pr₃ In(NEt₃). The protonNMR data are summarised below:

    ______________________________________                                        (ppm)                (Assignment)                                             ______________________________________                                        0.8 (triplet, 9H)    NCH.sub.2 CH.sub.3                                         1.1 (multiplet, 3H) InCH(CH.sub.3).sub.2                                      1.6 (doublet, 18H) InCH(CH.sub.3).sub.2                                       2.4 (quartet, 6H) NCH.sub.2 CH.sub.3                                        ______________________________________                                    

The i-Pr₃ In-NEt₃ adduct was further analysed for trace metal impuritiesusing ICP-ES. The only impurities detected were silicon (0.04 ppm w.r.tIn) and zinc (3.8 ppm w.r.t In).

Yield i-Pr₃ In(NEt₃)=8 g.

EXAMPLE 4

This example demonstrates the production of triisopropylindium usingdimethylethylamine as solvent.

A solution of i-PrMgBr in NMe₂ Et was prepared by the dropwise additionof i-PrBr (192 g, 1.6 mol) to a stirred suspension of Mg metal turnings(56 g, 2.3 mol) in NMe₂ Et (400 cm³).

This resulted in a vigorous exothermic reaction. After complete additionof the i-PrBr the reaction mixture was stirred for 4 hours at roomtemperature.

The solution of i-PrMgBr in NMe₂ Et was added dropwise, with stirring,to a suspension of InCl₃ (72 g, 0.3 mol) in pentane. This resulted in anexothermic reaction. After complete addition of the i-PrMgBr/NMe₂ Etsolution, the reaction mixture was boiled under reflux for 2 hours.

After removal of volatiles by atmospheric pressure distillation, (60°C.), the crude product was obtained by reduced pressure distillation(85-90° C.) into a receiver. Volatile impurities were removed from thecrude product by vacuum distillation (25° C.).

The pure liquid product was obtained by vacuum distillation (85-90° C.)into a receiver cooled to approx. -196° C.

The straw yellow liquid was identified using proton NMR spectroscopy asthe dimethylethylamine adduct of tri-isopropyl indium, iPr₃ In(NMe₂ Et).The proton NMR data are summarised below:

    ______________________________________                                        (ppm)                (Assignment)                                             ______________________________________                                        0.8 (triplet, 3H)    NCH.sub.2 CH.sub.3                                         1.0 (multiplet, 3H) InCH(CH.sub.3).sub.2                                      1.5 (doublet, 18H) InCH(CH.sub.3).sub.2                                       2.0 (singlet, 6H) NCH.sub.3                                                   2.3 (quartet, 2H) NCH.sub.2 CH.sub.3                                        ______________________________________                                    

The i-Pr₃ In-NMe₂ Et adduct was further analysed for trace metalimpurities using ICP-EAS. The only impurities detected were silicon (<1ppm) w.r.t In), and Zn(0.12 w.r.t In).

Yield i-Pr₃ In(NMe₂ Et)=81.7 g.0

We claim:
 1. A method of growing a semiconductor layer on a substratecomprising the steps of:delivering to the substrate, a metalorganiccompound of the formula MR₃, M being a metal and R being an alkyl group,prepared by reacting a Grignard reagent with a metal halide, and causingdeposition on the substrate of the metal from the metalorganic compound,wherein reaction of the Grignard reagent with the metal halide iscarried out in an amine solvent.
 2. A method as claimed in claim 1,wherein the metalorganic compound is used for said deposition in theform of amine adduct.
 3. A method as claimed in claim 1, wherein theGrignard reagent is prepared in an amine solvent.
 4. A method as claimedin claim 3, wherein the amine solvent used in preparing the Grignardreagent is the same as that used for the reaction thereof with the metalhalide.
 5. A method as claimed in claim 1, wherein the amine is atertiary amine.
 6. A method as claimed in claim 5, wherein the amine isselected from the group consisting of tertiary alkyl amines and tertiaryheterocyclic amines.
 7. A method as claimed in claim 1, wherein theamine is liquid at room temperature.
 8. A method as claimed in claim 1,wherein the amine has the following general formula: ##STR3## whereinR¹, R² and R³ are alkyl groups having from 1 to 4 carbon atoms andwherein R¹, R² and R³ are the same or two of R¹, R² and R³ are the same.9. A method as claimed in claim 1, wherein the amine is selected fromthe group consisting of triethylamine and dimethylethylamine.
 10. Amethod as claimed in claim 1, wherein the amine is selected from thegroup consisting of pyridine, 2H-pyrrole, pyrimidine, pyrazine,1,3,5-triazine and hexahydrotriazine.
 11. A method as claimed in claim1, wherein the Grignard reagent is prepared by reaction of magnesiumwith an alkyl halide, the alkyl group thereof being that required forthe metalorganic compound.
 12. A method as claimed in claim 1, whereinthe metalorganic compound is selected from the group consisting oftrialkyl aluminium, trialkyl gallium, trialkyl indium, trialkylphosphorus, trialkyl arsenic and trialkyl antimony.
 13. A method asclaimed in claim 1, wherein the alkyl groups are isopropyl groups.
 14. Amethod as claimed in claim 1, wherein the metalorganic compound is usedfor said deposition in the form of amine adduct having the formula:

    MR.sub.3.A

wherein M is a metal; R is an alkyl group; and A is a tertiaryheterocyclic amine or is a tertiary alkyl amine of the formula: ##STR4##where R¹, R² and R³ are alkyl groups having from 1 to 4 carbon atoms andwherein R¹, R² and R³ are the same or two of R¹, R² and R³ are the same.15. A method as claimed in claim 14, wherein the alkyl group of the MR₃is a C₁₋₅ straight or branched chain alkyl group.
 16. A method asclaimed in claim 14, wherein the alkyl group of the MR₃ is an isopropylgroup.
 17. A method as claimed in claim 1, wherein the metalorganiccompound or its amine adduct is delivered as a vapour to the substrateand pyrolysed thereon.
 18. A method as claimed in claim 1, wherein thesubstrate is a Group III-V single crystal.
 19. A method as claimed inclaim 18, wherein the substrate is gallium arsenide.
 20. A method asclaimed in claim 18, wherein the metalorganic compound is trialkylgallium.
 21. A method as claimed in claim 18, wherein the metalorganiccompound is triisopropyl gallium.
 22. A method as claimed in claim 1,wherein the metalorganic compound is triisopropyl gallium which isdelivered to the substrate with a Group V element precursor to form alayer having a gallium and the Group V element.
 23. A method as claimedin claim 22, wherein a Group III precursor in addition to thetriisopropyl gallium is also delivered to the substrate.
 24. A method asclaimed in claim 22, wherein the Group V precursor is arsine and asecond Group III precursor is also delivered to the substrate, thesecond Group III precursor being an AlH₃ amine adduct.